According to GSM standard operating procedures, a mobile terminal or station must first request a signaling channel from the network before the call setup can proceed. This signaling channel is used for the exchange of call setup related messages between the mobile terminal and the network. Since a large number of signaling messages must be exchanged between the mobile station and the network in order to setup a call, the speed of the signaling channel directly affects the connection setup delay.
In the prior art, to conserve wireless telephony bandwidth, the signaling channel assigned to the mobile station at the beginning of the call is a normally low speed signalling channel designated as a "SDCCH" channel. Once the call setup related signaling is complete, the network will assign a traffic channel to the mobile station. In general the traffic channel is a higher speed channel than the signaling channel which was originally assigned to that mobile station.
Depending upon the type of signaling channel used for setting up the call and the time when the traffic channel is assigned to the mobile station, there are two major call setup options presently being utilized in the industry. A first option is designated in the GSM standard as early assignment (EA) while a second GSM standard option is designated as very early assignment (VEA).
Under EA, the mobile station is initially assigned a standalone dedicated control channel (SDCCH). The SDCCH channel is a relatively slow channel which is intended for carrying signaling traffic only. As soon as the call "setup" message from the mobile station to the mobile switching center and the return "call proceeding" message are completed, the network assigns a full traffic channel, (TCH/F), to the mobile station if a voice or user generated data call has been detected by the network. This traffic channel remains idle until the user communication starts. If no voice call is detected by the network, the call is completed at the slow channel rate.
Under VEA, the mobile station is assigned a TCH/F channel at the very beginning of the call setup. This full traffic channel is initially configured or used in a mode called a signaling channel and it is used for the exchange of all call setup related signaling messages. After the setup message is received from the mobile station, the network modifies the transmission mode of the TCH/F channel to a speech channel for voice or a data channel for data if the communication is voice or user generated data respectively. This TCH/F channel stays idle until user communication starts.
It has been found that, depending upon many variables, the average call setup delay for mobile to mobile calls using EA is in the range of 7 to 15 seconds. This call setup delay is too long in the opinion of most network providers. When the VEA setup approach is used, the call setup delay, for a given network, can be typically reduced to almost half the delay that would have occurred using the EA setup approach. However, since a TCH/F channel is always assigned to the mobile station regardless of the type of service requested by the mobile station, the amount of user traffic that can be carried by the TCH/F channel is reduced, thus the call blocking probability for the network is increased and accordingly the network capacity is decreased.